Improve inter-document focus handling (#36649)

*Describe the changes that this pull request makes here. This will be
the commit message.*
rewritten the PR #28571
Implement
[Window#focus](https://html.spec.whatwg.org/multipage/#dom-window-focus),
[Window#blur](https://html.spec.whatwg.org/multipage/#dom-window-blur)
Testing: WPT
Fixes: #8981 #9421

---------

Signed-off-by: kongbai1996 <1782765876@qq.com>
Co-authored-by: yvt <i@yvt.jp>

components/shared/embedder/lib.rs

@@ -14,7 +14,7 @@ pub mod user_content_manager;
 mod webdriver;
 
 use std::ffi::c_void;
-use std::fmt::{Debug, Error, Formatter};
+use std::fmt::{Debug, Display, Error, Formatter};
 use std::path::PathBuf;
 use std::sync::Arc;
 
@@ -784,3 +784,76 @@ pub enum AnimationState {
     /// No animations are active but callbacks are queued
     NoAnimationCallbacksPresent,
 }
+
+/// A sequence number generated by a script thread for its pipelines. The
+/// constellation attaches the target pipeline's last seen `FocusSequenceNumber`
+/// to every focus-related message it sends.
+///
+/// This is used to resolve the inconsistency that occurs due to bidirectional
+/// focus state synchronization and provide eventual consistency. Example:
+///
+/// ```text
+/// script                            constellation
+/// -----------------------------------------------------------------------
+/// send ActivateDocument ----------> receive ActivateDocument
+///                             ,---- send FocusDocument
+///                             |
+/// focus an iframe             |
+/// send Focus -----------------|---> receive Focus
+///                             |     focus the iframe's content document
+/// receive FocusDocument <-----'     send FocusDocument to the content pipeline --> ...
+/// unfocus the iframe
+/// focus the document
+///
+/// Final state:                      Final state:
+///  the iframe is not focused         the iframe is focused
+/// ```
+///
+/// When the above sequence completes, from the script thread's point of view,
+/// the iframe is unfocused, but from the constellation's point of view, the
+/// iframe is still focused.
+///
+/// This inconsistency can be resolved by associating a sequence number to each
+/// message. Whenever a script thread initiates a focus operation, it generates
+/// and sends a brand new sequence number. The constellation attaches the
+/// last-received sequence number to each message it sends. This way, the script
+/// thread can discard out-dated incoming focus messages, and eventually, all
+/// actors converge to the consistent state which is determined based on the
+/// last focus message received by the constellation.
+///
+/// ```text
+/// script                            constellation
+/// -----------------------------------------------------------------------
+/// send ActivateDocument ----------> receive ActivateDocument
+///                             ,---- send FocusDocument (0)
+///                             |
+/// seq_number += 1             |
+/// focus an iframe             |
+/// send Focus (1) -------------|---> receive Focus (1)
+///                             |     focus the iframe's content document
+/// receive FocusDocument (0) <-'     send FocusDocument to the content pipeline --> ...
+/// ignore it because 0 < 1
+///
+/// Final state:                      Final state:
+///  the iframe is focused             the iframe is focused
+/// ```
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Default,
+    Deserialize,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    PartialEq,
+    Serialize,
+    PartialOrd,
+)]
+pub struct FocusSequenceNumber(pub u64);
+
+impl Display for FocusSequenceNumber {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
+        Display::fmt(&self.0, f)
+    }
+}